This invention relates to a vinylidene fluoride copolymer film, more particularly, to a film of a vinylidene fluoride copolymer (hereinafter frequently referred to as "VDF copolymer") to which transparency is imparted by film formation at normal temperature or around normal temperature and a process for producing the same.
Vinylidene fluoride resins (hereinafter frequently referred to as "VDF resins"), typically, polyvinylidene fluoride (hereinafter referred to as "PVDF"), are excellent in weathering resistance, staining resistance, water resistance, etc. and they are known to be useful as a material of film by itself or a coating of a film product. However, there has been an obstacle to wide utilization of the VDF resin film, that is, the difficulty in forming a transparent homogeneous film. For example, among various methods proposed concerning application of coating on a shaped body, Japanese Patent Publication No. 4176/1963 discloses a method wherein a PVDF solution is applied on a shaped body, then dried first at a high temperature of 100.degree. to 150.degree. C. and further heated to a temperature of the melting point of the PVDF or higher to form a transparent film. Such a high temperature drying is carried out because, in the case of mere drying at normal temperature, the coating obtained is under a state of merely pressure bonded powder, which is opaque in white and fragile and can hardly be called a film. However, when considering application for uses requiring excellent characteristics such as weathering resistance, etc. of VDF resin films, for example, coatings on already built structures such as bridges, towers, etc., or large scale equipments, including transportation means such as ship, vehicles, etc., the necessity of application at such a high temperature has markedly impeded utilization of VDF resin films.
Accordingly, in order to obtain a homogeneous and transparent VDF resin film by film formation at around normal temperature, various proposals have been made. One typical method is disclosed by Japanese Patent Publicaion No. 14536/1971. According to this method, VDF resin is formed into a film by casting or others, on the basis of a discovery that existence of both .alpha.-phase and .beta.-phase crystals in a mixture gives a transparent film, as contrasted to the .beta.-phase crystal of VDF resin, which is rapidly crystallized to form spherulites thus tending to give a film with turbidity over the whole surface. On the other hand, with the change in composition from PVDF to a VDF copolymer, the crystal form is shifted to .beta.-phase and therefore, in this method, PVDF homopolymer or a VDF copolymer containing 95% or more of vinylidene fluoride is employed. Also, even if such a polymer may be employed, at a critical temperature determined depending on the kind of the solvent or a higher temperature, due to solvation action, the crystal form will be shifted to the .beta.-phase which can readily be solvated. For this reason, it has been required to dissolve the polymer in a solvent at a temperature lower than the critical temperature followed by casting. The critical temperature is, for example, about 40.degree. C., when dimethylacetamide is employed as the solvent, while it is about 70.degree. C. when dimethylformamide is employed. However, restriction to a lower temperature than the relatively low critical temperature in forming a solution as mentioned above leads to use of the solvent under the conditions where its solvent ability cannot effectively be utilized. This cannot always be advantageous in view of a film forming step. Also, even in a solution prepared at a temperature lower than the critical temperature, when maintained under the state as prepared for a long time, transition to the .beta.-phase structure which is readily solvated will progress to give also only intransparent films. Because of these reasons, this method of the prior art involves a drawback that it is difficult to obtain transparent films of constant quality.
Further, U.S. Pat. No. 4,335,238 to Moore et al. discloses a terpolymer consisting of 30-81% by weight of vinylidene fluoride, 12-48% by weight of tetrafluoroethylene and 7-24% by weight of hexafluoropropylene. This terpolymer provides a transparent film when formed around room temperature. However, such a copolymer having a relatively large content of a branched monomer such as hexafluoropropylene shows an elastic nature because a large space occupied by the side branch hinders the crystallization of the polymer. While the polymer having substantially no crystallinity can be transparent, the softening point of the polymer becomes 130.degree. C. or below, whereby an isolated film or a coating film thereof cannot be applied to outdoor use because it becomes too soft and is liable to cause deformation when irradiated by the sunlight.